The present invention relates to medical procedures and apparatus to perform medical procedures to determine the flow rate of blood. In particular, the invention relates to the determination of cardiac output characteristics for diagnosis purposes.
The determination of cardiac output, or measurement of the blood volumetric output of the heart is substantially important for a variety of medical situations. Healthcare professionals utilize such information along with a number of additional pulmonary factors to evaluate the condition of their subject""s heart. Even with the variety of approaches developed for measuring this output, each exhibit certain limitations and/or inaccuracies. The volumetric aspect of cardiac output provides information about the sufficiency of oxygen delivery to tissue or the oxygenation of the tissue. When used in combination with other measurements it provides important status and evaluation information of the cardiovascular system.
Methods for determining cardiac output as the thermodilution method are discussed, for example in U.S. Pat. Nos. 3,651,318, 4,217,910, and 4,236,527. As conventionally employed, this method involves either injecting a bolus of liquid into the bloodstream at a temperature which is cooler or warmer (usually cooler) than blood temperature, or heating a segment of the blood indirectly with electrical resistance heaters, and monitoring the temperature deviation of the blood as a function of time at a place downstream from the place at which the temperature deviation is caused. The area under the resulting temperature deviation vs. time curve (known as the thermodilution curve) is a measure of the rate at which the heart is pumping blood (usually expressed in liters per minute). If cardiac output is high, the area under the thermodilution curve will be relatively small in accordance with the well-known Stewart-Hamilton relationship. Conversely, if cardiac output is low, the area under the thermodilution curve will be relatively large.
Currently, the more accepted approach for deriving cardiac output values is an indicator dilution technique which takes advantage of refinements made earlier in pulmonary catheter technology. The standard of cardiac output measurement from pulmonary artery catheterization are described in for example, in U.S. Pat. Nos. 3,915,155, 3,726,269 and 3,651,318 involve periodic injection into the subject""s bloodstream of a bolus, during which thermodilution measurements are performed to determine cardiac output. Such techniques cannot generally be used for continuous monitoring. Moreover, such catheterization techniques pose significant risk to the subject, including malignant arrhythmias, pulmonary artery rupture, and in rare cases, death. However since knowledge of cardiac output is crucial in the care of critically ill subjects, as well as subjects with chronic heart disease requiring monitoring of medication work has been underway to develop less invasive apparatus and methods for monitoring cardiac output.
Advances in the art now require only central venous and arterial catheters as opposed to such invasive methods. Additionally, processes and devices have been developed to determine the fill level of the circulatory system of a patient disclosed in WO93/21823. Additionally, WO93/21823 provides a process for determining the end diastolic heart volume, the pulmonary blood volume, the extravascular thermovolume and/or the global cardiac function index. Extravascular thermovolume correlates, if there is no significant perfusion defect in the lungs (e.g., pulmonary embolism), closely to the degree of extravascular lung water. However, the clinical value of that measurement has not been shown explicitly yet.
In a typical procedure, a cold bolus of saline at ice or room temperature in an amount of about 5-10 milliliters is injected through the catheter as a measurement procedure which will require about two minutes to complete. For purposes of gaining accuracy, this procedure is repeated three or four times and readings are averaged. Consequently, the procedure requires an elapsed time of 4-5 minutes. In general, the first measurement undertaken is discarded inasmuch as the catheter will have resided in the bloodstream of the body at a temperature of about 37xc2x0 C. Accordingly, the first measurement procedure typically is employed for the purpose of cooling the dilution channel of the catheter, and the remaining measurements then are averaged to obtain a single cardiac output value. Thus, up to about 40 ml of fluid is injected into the intravascular system of the patient with each measurement which is undertaken. As a consequence, this procedure is carried out typically only one to two times per hour over a period of 24 to 72 hours. While practitioners would prefer that the information be developed with much greater frequency, the procedure, while considered to be quite accurate, will add too much fluid to the cardiovascular system if carried out too often.
Of course, the accuracy of the procedure is dependent upon an accurate knowledge of the temperature, volume, and rate of injection of the liquid bolus. Liquid volume measurements during manual infusions are difficult to make with substantial accuracy. For example, a syringe may be used for injecting through the catheter with the result that the volume may be identified only within several percent of its actual volume. Operator error associated with volume measurement and rate of injection also may be a problem. Because the pulmonary catheters employed are somewhat lengthy (approximately 30 to 40 inches), it is difficult to know precisely the temperature of the liquid injectate at the point at which it enters the bloodstream near the distal end of that catheter. Heat exchange of the liquid dispensing device such as a syringe with the catheter, and the blood and tissue surrounding the catheter upstream of the point at which the liquid is actually released into the blood may mean that the injectate temperature is known only to within about five percent of its actual temperature. Notwithstanding the slowness of measurement and labor intensity of the cold bolus technique, it is often referred to as the xe2x80x9cgold standardxe2x80x9d for cardiac output measurement by practitioners. In this regard, other techniques of determining cardiac output typically are evaluated by comparison with the cold bolus approach in order to determine their acceptability.
Another technique of thermodilution to measure cardiac output employs a pulse of temperature elevation as the indicator signal. In general, a heating coil is mounted upon the indwelling catheter so as to be located near the entrance of the heart. That coil is heated for an interval of about three seconds which, in turn, functions to heat the blood passing adjacent to it. As is apparent, the amount of heat which can be generated from a heater element is limited to avoid a thermocoagulation of the blood or damage to tissue in adjacency with the heater. This limits the extent of the signal which will be developed in the presence of what may be considered thermal noise within the human body. In this regard, measurement error will be a result of such noise phenomena because of the physiological blood temperature variation present in the body. Such variations are caused by respirations, coughing, and the effects of certain of the organs of the body itself. For further discussion see Afonzo, S., et al., xe2x80x9cIntravascular and Intracardiac Blood Temperatures in Man,xe2x80x9d Journal of Applied Physiology, Vol. 17, pp 706-708, 1962 and U.S. Pat. No.4,595,015.
This thermal noise-based difficulty is not encountered in the cold bolus technique described above, inasmuch as the caloric content of a cold bolus measurement is on the order of about 300 calories. By contrast, because of the limitations on the amount of heat which is generated for the temperature approach, only 15 or 20 calories are available for the measurement. Investigators have attempted to correct for the thermal noise problem through the utilization of filtering techniques, for example, utilizing moving averages over 6 to 12 readings. However, where such corrective filtering approaches are utilized, a sudden downturn in the hemodynamic system of a patient will not be observed by the practitioner until it may be too late. The effective measurement frequency or interval for this technique is somewhat extended, for example about 10 minutes, because of the inaccuracies encountered. In this regard, a cardiac output value is achieved only as a consequence of a sequence of numerous measurements. In general, the approach does not achieve the accuracy of the above-discussed cold bolus technique. Thermodilution techniques involving the use of electrical resistance heaters are described, for example, in U.S. Pat. Nos. 3,359,974, 4,217,910, 4,240,441 and 5,435,308.
Other approaches to the elimination of an injectant in thermodilution procedures have been, for example, to introduce the thermal signal into the flowing blood by circulating a liquid within the catheter, such liquid preferably being cooler than the blood temperature. See in this regard, U.S. Pat. No.4,819,655. While, advantageously, no injectant is utilized with such procedure, the method has the disadvantage that only a limited thermal signal is available as compared with the cold bolus approach, and, thus, the measurement is susceptible to error due to physiological temperature variations. As another example, a technique has been proposed wherein a stochastic excitation signal present as a series of thermal pulses of varying duration is asserted within the bloodstream, and the resultant output signal downstream, now present as blood temperature variation, is measured. The blood flow rate then is extracted by cross-correlating the excitation signal and measured output signal. See U.S. Pat. No. 4,507,974.
The current state of the art in performing transpulmonary thermodilution measurements is to inject a bolus of thermal indicator. The used injectate volume in most application is 10 ml or, as a guideline, 0.15 ml/kg. Cardiac output is calculated from an arterial thermodilution curve in the usual way using the Stewart-Hamilton algorithm. For a cold bolus injection algorithms to derive the cardiac output are based on the Stewart-Hamilton equation:   CO  =                              V          i                ⁡                  (                                    T              B                        -                          T              i                                )                    ⁢              K        1            ⁢              K        2                    ∫              Δ        ⁢                  xe2x80x83                ⁢                              T            B                    ⁡                      (            t            )                          ⁢                  ⅆ          t                    
where TB is the blood temperature, Ti is the indicator temperature, Vi is the indicator volume, K1 and K2 are constants to consider the specific measurement setup and xcex94TB(t) is the blood temperature as a function of time with respect to the baseline blood temperature ((TB). To obtain cardiac output in liters per minute, the area under the thermodilution curve has to be integrated. Then the amount of extravascular thermovolume is computed via the determined cardiac output and the two parameters mean transit time (MTt) and exponential downslope time (DSt).
A typical, ideal transpulmonary thermodilution curve is shown in FIG. 1. In this example, the inverted blood temperature (the peak represents a colder blood temperature) after a cold bolus injection is presented. Using an indicator, warmer than the blood temperature results in a principally similar curve. Cardiac output (CO) is derived from the amount of indicator used and the area under the shown curve.
The present invention addresses the need for improved methods for determining the cardiac output of a subject by thermodilution measurements. As an advantage over the present state of the art the invention provides a method for increasing the accuracy of thermodilution measurements by means of biofeedback and adjustment.
In one embodiment of the invention, the information is obtained by providing a predetermined amount of thermal indicator in a blood vessel having a temperature different from the temperature of subject""s blood, thus exhibiting an indicator temperature difference. A thermodilution curve is then determined by measuring the temperature of subject""s blood at a second place downstream from the injection site as a function of time. Cardiac output (CO) is then determined from the thermodilution curve to provide the extravascular thermovolume. A new amount of thermal indicator and/or a new thermal indicator temperature difference can then be determined according to the new thermovolume. The process can then be repeated with the new amount of indicator and/or new indicator temperature difference to provide more accurate results.
In another embodiment, the invention also provides a means where the predetermined amount of thermal indicator is provided as an indicator liquid.
In another embodiment, the invention also provides for the cardiac output to be determined by transpulmonary thermodilution and when the extravascular thermovolume correlates to extravascular lung water volume. Extravascular thermovolume correlates, if there is no significant perfusion defect in the lungs (e.g., pulmonary embolism), closely to the degree of extravascular lung water. However, the clinical value of the measurement has not been shown explicitly yet.
In another embodiment, the invention also provides for the new amount of indicator and/or new indicator temperature difference to be increased or decreased according to the amount of said determined extravascular thermovolume.
In another embodiment, the invention also provides for the extravascular thermovolume to be determined from a mean transit time of said thermal indicator required by the thermal indicator to travel from the first place to the second place, from the downslope of the thermodilution curve and from the cardiac output.
Another embodiment of the invention provides an apparatus for determining the cardiac output of a subject by thermodilution measurements by providing a means of delivering at a first place a predetermined amount of thermal indicator in a blood vessel of the subject. The thermal indicator would have a temperature different from the temperature of subject""s blood, to express an indicator temperature difference. Also, the apparatus provides a temperature sensor for measuring the temperature of subject""s blood at a second place downstream of the first place as a function of time to determine a thermodilution curve. A computer is connected to the temperature sensor for determining cardiac output and an extravascular thermovolume from the thermodilution curve. The computer utilizes the measurements gathered and determines a new amount of indicator and/or a new indicator temperature difference according to the thermovolume. Then the computer controls and provides means of supplying a new amount of thermal indicator/temperature difference in the blood vessel and controls the temperature sensor to measure the temperature of subject""s blood as a function of time at the second place.
In another aspect of the invention, the apparatus may utilize an injector as the means of delivery.